The invention relates generally to electrical device packaging and more to packaging of electrical devices requiring energy transmission to and/or from device active areas.
As the need for smaller, lighter, portable and/or mobile equipment increases in applications such as medical imaging, consumer electronics, and communication devices, the need for high density device and system level packaging likewise increases. High density packages would be most beneficial if adapted for interconnecting hundreds of input and output pads and providing an unobstructed path for transmission and/or absorption of energy directed to and/or from the active area of a packaged device.
For example, ultrasound imaging equipment uses active areas of electrical devices to transmit and receive hundreds of acoustic signals which can be used to create images of patient anatomy. The active areas of these devices must be substantially free of material that would attenuate or distort the signals. Accordingly, ideal packaging technology would be capable of interconnecting high pin count acoustic devices with fine pitched (less than 0.2 millimeter, for example) input/output pads and providing an open window over the active area of each packaged device. Preferably, such an open window would offer protection from damage to the device active area during the packaging process.
Cole et al., U.S. Pat. No. 5,527,741, issued 18 Jun. 1996, describes a method for fabricating a circuit module by forming a flexible interconnect layer of multiple layers of metallized polymer film, attaching a circuit chip to the film with an adhesive, forming vias through the flexible interconnect layer, and applying metallization to couple chip pads through the vias. Such flex-based packaging has been useful for high pin count devices with fine pitch but has resulted in embodiments wherein the entire active area of the device is covered by the flexible interconnect layer. Thus such packaging has been limited with respect to optical or acoustic sensors that cannot operate efficiently when covered by material which attenuates or distorts energy transmission.
As described in Kornrumpf et al., U.S. Pat. No. 5,355,102, issued 11 Oct. 1994, ablating dielectric material over the center of a device may adversely affect the device. To protect microwave devices, Kornrumpf et al. recommended placing a small piece of polytetrafluoroethylene or other non-laser ablatable material on top of the active area of the device to cover the region from which the dielectric is to be removed after completion of the high density interconnect fabrication process, and, after such fabrication, cutting the dielectric layers and removing the piece of polytetrafluoroethylene.